This invention relates to an apparatus and a process for deforming materials, and, more particularly, to an apparatus and a process for continuously and progressively forming composite materials containing continuous fibers.
A composite material combines two or more other materials into a single integrated material structure, in a manner whereby the combined materials retain their original identities. One of the best known, commercially most important types of composites is the fiber composite material formed of long, substantially continuous high-strength fibers incorporated into a metallic or a polymeric matrix. These composites typically have large numbers of fibers embedded in the matrix. The fibers are usually selected to be strong, but are of low elongation to failure. Examples of the materials used in high-strength fibers are carbon, graphite, boron and silicon carbide. Composites with discontinuous fibers are made for some applications, but the best strength properties are obtained when the fibers are continuous, with aspect ratios of at least 10.
The matrix holds the fibers in the proper orientation and protects from from external damage. Polymeric matrix materials fall into two general classes, thermosetting and thermoplastic. Most thermosetting polymers are not formable by deformation processes, and the present invention doe not relate to such nondeformable polymers. Like metals, thermoplastic polymers can be plastically deformed, at sufficiently high temperatures, to a permanent change in shape without fracture.
The present invention relates to fiber composite materials having substantially continuous fibers in a plastically deformable matrix. At the present time one of the most important types of such composites is prepared with carbon fibers embedded in a polyetheretherketone (PEEK) matrix. The development of such fiber composite materials is continuing, with new combinations having improved properties emerging each year.
The fiber composite materials have exceptionally good strength and modulus properties with low weight, which makes them attractive in a variety of applications. Because the cost of such composites in the past has been high, their use has been limited to specialized applications such as certain aircraft and spacecraft components, golf clubs, tennis rackets and sailboat masts. Even though composite materials would offer significantly improved performance in many other applications, the high cost of preparing the basic material and then fabricating it into the final shape has inhibited widespread use of the material.
The most commonly used approach to making pieces of the composite material is to lay up tapes of fiber-containing matrix material into flat pieces, and then to consolidate the tapes by pressing the flat pieces in a large press. To achieve good strength and stiffness properties in the finished material, the fibers in the layers may be oriented in various directions within the plane of the piece. The consolidated laminate therefore consists of discrete layers. Within each layer there is an intimate mixture of fiber and matrix material, but between layers there is often some additional matrix material.
The press used to consolidate the strips can utilize flat platens to press the layers of composite material, to produce a flat plate of the composite material. Flat plates have limited uses, because most components are not flat and contain curved sections. Shaped pieces of composite can be made by substituting fixed dies for the flat platens, so that the composite is consolidated to the desired final shape. Alternatively, pieces of the composite can be consolidated as flat plates, and then reshaped using fixed dies corresponding to the overall final shape of the component, in a separate die-forming operation. This approach is termed post forming.
The fabrication of composite structures using fixed dies has significant drawbacks and limitations, both from technical and economic standpoints. Forming in a press having dies for platens often results in gaps and voids in the structure, particularly between layers, because of the difficulties in achieving the proper uniform layup of the tapes prior to pressing and a uniform pressing pressure during die forming. The post-forming approach may produce high compressive stresses in the fibers in the concave-going side of bends, resulting in buckling and misalignment of the fibers and unsatisfactory mechanical properties.
Economic drawbacks to the use of the die-pressing approach are equally significant. If the piece of composite structure is large, the die must be large and the die press must also be large, resulting in high costs for the preparation of the die and high capital costs in pressure and furnaces for heating the die during forming. If only a few parts are to be fabricated, the cost of making the die may be a large fraction of the total cost. Specialized dies may requires lead times of months to prepare, requiring that they be designed and ordered long before the actual manufacturing is to occur. While the ordering of long-lead time dies is not an insurmountable obstacle, failure of a die after it is received or the need to change the die design do create significant problems. The science and art of composite materials has not progressed to the point that the structures and dies can be designed without trial and error in some cases. The long lead times for obtaining dies significantly inhibits the ability of engineers to change the designs of the composite structures for either structural or manufacturing reasons after the dies are received, so that optimization is difficult with the present manufacturing technology.
Accordingly, there exists a need for an improved approach to the manufacturing of composite material structures having bent or curved sections. Such an apparatus and process should allow the manufacture of structures having excellent properties, and in particular should avoid the formation of voids or misaligned fibers in the structure, problems inherent in the present techniques. The size of the forming apparatus would desirably be small, minimizing capital costs and allowing the forming of parts larger than the apparatus itself. A highly desirable feature would be a degree of adjustability so that many different shapes of structures could be formed with the one apparatus, increasing its versatility and eliminating the lead time normally spent in preparing new dies for each new part. The present invention fulfills this need, and further provides related advantages.